1. Field of the Invention
The present invention relates to a surface acoustic wave device having a thin film made of at least one of a silicon nitride film (SiN film), a silicon oxide film (SiO film), and a silicon oxide nitride film (SiON film) and also relates to a method of manufacturing such a surface acoustic wave device.
2. Description of the Related Art
SiN films, SiO films, and SiON films have been widely used in surface acoustic wave devices (hereinafter, referred to as a SAW device). For example, these films are used as functional films for finely adjusting the frequencies of SAW devices, protecting the devices to improve the weather resistances thereof (moisture-proofing films), improving the temperature characteristics thereof, etc. These functional films determine the performances of the SAW devices. Thus, the manufacture of these films is required to be stable and highly-reproducible.
In recent years, to cope with the reduction in size and weight of SAW devices, resin materials such as epoxy resin have been more frequently used as parts of packages for SAW devices. The resin materials are water-permeable. Accordingly, in the above-described use, water which permeates through a resin material portion of such packages causes an interdigital electrode portion (hereinafter, referred to as IDT), which is generally made of an Al material, to be corroded. This deteriorates the characteristics of the SAW device.
Therefore, protection-film formation techniques for improving the moisture-proof property have been more important. Regarding methods for forming such films, methods that are generally called sputtering methods such as RF sputtering, DC sputtering, RF magnetron sputtering, and other such methods are used for forming the SiO films. The SiN films and the SiON films are formed by a plasma-CVD (chemical Vapor Deposition) method (hereinafter, briefly referred to as P-CVD) and other suitable methods.
However, when the above-described thin films are formed as functional films on the chips (IDTs on piezoelectric substrates) of SAW devices, respectively, problems are caused in that the electrical characteristics of the SAW devices are deteriorated. Especially, increases in the insertion loss is one of the reasons why the film thicknesses can not be increased. For example, if a protection film having a film-thickness at which the insertion loss is allowable is formed as a functional film, problematically, the protection film will be insufficient as a functional film for improving the moisture-proof property.
For example, when an SiN film with a thickness of 10 nm is formed on a SAW filter by a P-CVD method to improve the moisture-proof property of the filter, the insertion loss is increased by about 0.3 dB. The increased insertion loss is caused by the fact that the SIN film formed by the P-CVD method is not tight.
Furthermore, in the case of lithium niobate (LiNbO3) used for a piezoelectric substrate of a SAW device, the temperature of the piezoelectric substrate can not be increased when the film is formed. Accordingly, the P-CVD method for film-formation which requires heating at a temperature of about 300xc2x0 C. to 400xc2x0 C. cannot be applied to the piezoelectric substrate made of LiNbO3.
In order to overcome the problems described above, preferred embodiments of the present invention provide a SAW device having as a functional film at least one of an SiN film, an SiO film, and an SiON film, by which the characteristics of the SAW device are prevented from being deteriorated, the film having a tight structure, and also provide a method of manufacturing the SAW device by which the functional film can be reliably formed.
According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate, interdigital electrode portions disposed on the piezoelectric substrate, a functional film including at least one of a silicon nitride film, a silicon oxide film, and a silicon oxide nitride film and disposed on at least a portion of the interdigital electrode portions, and a package including the piezoelectric substrate having the interdigital electrode portions and the functional film, at least a portion of the package being made of a moisture-permeable material, the functional film being formed by an electron cyclotron resonance sputtering method (hereinafter, the electron cyclotron resonance is referred to as ECR).
Preferably, the package including the piezoelectric substrate having the interdigital electrode portions and the functional film defines a structure in which the piezoelectric substrate mounted by a flip-chip bonding method, and the piezoelectric substrate is sealed with a moisture-permeable resin.
Also, preferably, the functional film is formed at a room temperature.
The functional film, which is preferably formed by an ECR sputtering method, has a tight-structure, so that the weather resistance is reliably improved. Moreover, according to the ECR sputtering method, heating of the substrate is not required for formation of the functional film. For example, the functional film can be formed at a room temperature. Thus, preferred embodiments of the present invention may be also applied to an LiNbO3 substrate of which the characteristics will be deteriorated by heating. Furthermore, the pyroelectric breakdown of the piezoelectric substrate, which may be caused by heating of the IDT, is reliably prevented.
The resistance to environmental conditions of the device is greatly improved according to preferred embodiments of the present invention. Thus, a material which is inexpensive and can be reduced in size, but is moisture-permeable such as epoxy resins or other suitable material can be used, e.g., for at least a portion of the package. Thus, the time-dependent deterioration of the characteristics of the device, which may be caused by permeated water, is reliably prevented. Moreover, the cost of the device is reduced.
In some cases, adhesion of an SiN film to a piezoelectric substrate is inferior to that of an SiO film, due to the difference between the coefficients of linear expansion thereof. Thus, according to another preferred embodiment of the present invention, the functional film includes the silicon nitride film, and the nitrogen concentration is changed in the film-thickness direction. More preferably, for the functional film having a nitrogen concentration changed as described above, the nitrogen concentration on the front surface side is larger than that on the interdigital electrode portion side. Also, preferably, in the functional film, the SiN film is provided on the front surface side, and the SiO film is located on the IDT side.
Improvements to the moisture-proof property and that of the adhesion to the piezoelectric substrate can be achieved by changing the nitrogen concentration of the functional film including an SiN film in the film-thickness direction, for example, by setting the nitrogen concentration in such a manner that the concentration on the front surface side is larger than that on the IDT side.
Preferably, the functional film includes the silicon nitride film, and the silicon nitride film preferably has a film-thickness of about 3 nm or greater, and the film thickness is preferably about 2.0% or less of the wavelength of a surface acoustic wave generated in the interdigital electrode portions.
Accordingly, a high moisture-proof property is reliably achieved.
Also, preferably, the functional film includes a silicon oxide film in the lowermost layer (on the IDT side), a silicon oxide nitride film in an intermediate layer, and a silicon nitride film in the uppermost layer.
The intermediate layer may have a multi-layer structure in which at least one silicon nitride film and at least one silicon oxide film are laminated to each other.
According to the above-described structure including the SiN film in the uppermost layer, a high moisture-proof property is reliably achieved.
To solve the above-described problems, according to another preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate, interdigital electrode portions disposed on the piezoelectric substrate, a functional film including a silicon nitride film and disposed on at least a portion of the interdigital electrode portions by an electron cyclotron resonance sputtering method, and a package including the piezoelectric substrate having the interdigital electrode portions and the functional film, at least a portion of the package being made of a moisture-permeable material, the functional film having a nitrogen concentration that varies in the film-thickness direction.
In this surface acoustic wave device, improvement of the moisture-proof property and that of the adhesion to the piezoelectric substrate are also achieved, similarly to the above-described surface acoustic wave device according to the first preferred embodiment of the present invention.
A silicon oxide film for frequency-adjustment may be disposed on the functional film. With the silicone oxide film, the frequency of the IDT on the piezoelectric substrate can be finely adjusted while the resistance to environmental conditions is not deteriorated.
To solve the above-described problems, according to a third preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave device which includes interdigital electrode portions on a piezoelectric substrate including the step of forming a functional film for improving the resistance to environmental conditions of the device including at least one of a silicon nitride film, a silicon oxide film, and a silicon oxide nitride film on at least a portion of the interdigital electrode portions by an ECR sputtering method.
By the above-described method, the functional film including at least one film made of SiN, SiO, and SiON which is tight and has a high purity can be formed, due to the use of the ECR sputtering method. Thus, a SAW device can be provided, in which deterioration of the characteristics is prevented, which may be caused when other methods are used. Moreover, for such SAW devices that cannot be film-formed with SiN, SiO, and SiON by a prior art method, the film-formation is enabled. A SAW device having a high reliability and improved characteristics is provided.
Especially, for SAW devices which have been used in recent years and of which a portion of the packages are made of a moisture-permeable material, SiN films or other suitable films can be formed without deterioration of the characteristics. Thus, a SAW device which has superior electrical characteristics, a high reliability, and is inexpensive is reliably provided.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.